1. Field of the Invention
The present invention relates to a touch panel that may be inputted with information by contact or approach of a user with a finger or the like, and particularly relates to a method of driving a touch panel detecting touch based on change in capacitance, and a touch panel and a display apparatus with a touch detection function using the method each.
2. Description of Related Art
A display device is recently noticed, in which a contact detector, so-called touch panel, is mounted on a display device such as liquid crystal display device, and various button images are displayed on the display device so as to allow information input as a substitute for typical mechanical buttons. While several types of touch panels, including an optical type and a resistance type, exist, a capacitance-type touch panel is particularly expected for a mobile terminal because of a relatively simple structure and low power consumption. However, in the case of the capacitance-type touch panel, a human body acts as an antenna receiving noise (hereinafter, called disturbance noise) caused by an inverter fluorescent lamp, an AM broadcast wave, an AC power supply and the like, and the noise may be transmitted to the touch panel, causing malfunction.
The malfunction is due to a fact that a signal on presence of touch (hereinafter, called touch signal) generated by contact or approach of a user with a finger is hardly distinguished from disturbance noise. The touch signal is typically A/D converted (sampled) by a detection circuit with folding noise being generated. Specifically, when a signal having a frequency component higher than a Nyquist frequency is sampled, the signal is folded with the Nyquist frequency as a reference, and outputted as a signal having a frequency component lower than the Nyquist frequency. Therefore, when disturbance noise having a frequency component higher than the Nyquist frequency is inputted to the detection circuit, frequency of such a noise component is equivalent to be in a frequency band equal to or lower than the Nyquist frequency. Thus, the disturbance noise is hardly distinguished from a touch signal to be normally detected in the frequency band equal to or lower than the Nyquist frequency.
Thus, for example, United States Patent Application Publication 2007/0257890 proposes a method, in which when a touch signal is detected, the touch signal being in synchronization with a signal (hereinafter, drive signal) driving a capacitance-type touch panel, a plurality of drive signals having different frequencies are used to select a condition, at which a touch signal is hardly affected by disturbance noise, for detection of the touch signal. In the method, a detection circuit uses the drive signals having different frequencies to sample a touch signal at a plurality of sampling frequencies corresponding to the drive signals, which inevitably leads to change in Nyquist frequency. That is, a folding reference is changed. Therefore, when disturbance noise having a high frequency component compared with the Nyquist frequency is inputted to the detection circuit, a frequency of a noise component equivalent to the disturbance noise, the noise component appearing in a frequency band not higher than the Nyquist frequency, is changed depending on the Nyquist frequency. In contrast, a frequency of a touch signal component is not changed even if the Nyquist frequency is changed. Therefore, the Nyquist frequency is selected such that the equivalent noise component may be distinguished from the touch signal component, so that the disturbance noise may be distinguished from the touch signal.